Hormonal Profiles in Summer and Winter Shoot Apices of Five Aquatic Plant Species

Hormonal profiles in winter organs of aquatic plants, formed in response to temperature decrease and short days, are largely unknown. We compared the hormonal levels in summer and winter shoot apices/fronds of five aquatic species with insufficiently known overwintering strategies with their dark respiration rates as a criterion for dormancy. Miniature samples of shoot apices/fronds were extracted and analyzed by UHPLC-ESI-MS/MS. Dark respiration of summer and winter shoot apices was measured using a Clark-type oxygen sensor. A simple growth test was conducted on Lemna trisulca and Ricciocarpos natans to ascertain their strength of dormancy. Variable respiration rates of turions and winter apices/fronds revealed marked interspecific differences in levels of dormancy. The winter total cytokinin (CK) content usually resembled the summer one or was significantly lower, with great interspecific differences. Active CKs decreased by about two times in winter. Also, the content of the active auxin IAA was usually lower than in summer, but in L. trisulca it rose 4.5 times. Increased abscisic acid (ABA) winter content (11-3124 nmol kg_DW ^-1) confirmed its significant accumulation in four species studied, except for Utricularia inflata. Generally, mature dormant turions have low dark respiration rates, in comparison with non-dormant winter shoot apices/fronds, at least on a dry-weight basis. Deviations from the rule correlate with the power of turion dormancy, which is variable among species and represents a continuous gradient from dormancy to quiescence. Concerning overwintering strategies in aquatic plants, the hormonal regulation of the formation of either turions or non-dormant winter apices is not universal.